JP2013048453A - Frequency offset compensation apparatus and method, and optical coherent receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a frequency offset compensation apparatus to be built in a phase offset compensation apparatus of an optical coherent receiver.SOLUTION: An optical coherent receiver includes a frequency offset estimation apparatus. The frequency offset estimation apparatus estimates a change in a phase offset due to a frequency offset of a base band digital electric signal of the optical coherent receiver. A frequency offset compensation apparatus includes: a first integration unit for integrating the change in the phase offset, and obtaining an additive inverse of the phase offset of a single symbol time; a multiplier for multiplying the change in the phase offset by the average length of a phase offset compensation apparatus,; and a second integration unit for integrating an output of the multiplier, and obtaining an additive inverse of the output of the multiplier.

Description

  The present invention relates to optical communication, and more particularly to an optical coherent receiver.

  As demand for capacity and flexibility of optical communication systems increases, coherent optical communication technology becomes more and more important. Compared to incoherent technology (eg, on-off-keying (OOK)) or self-coherent technology (eg, π / 4 DQPSK modulation (Differential Quadrature Phase Shift Keying)), coherent technology is a 3 dB optical signal pair. The transmission capacity can be increased by adopting modulation technology (for example, quadrature amplitude modulation (QAM)) with noise ratio (OSNR) gain and better efficiency, and the channel by adopting electrical equalization technology conveniently It is possible to cope with the change of the above, and there are advantages such as cost reduction.

  The coherent reception requires that the frequency and phase of the local oscillation and the phase of the carrier wave completely coincide, that is, the phase difference between the local oscillator and the carrier wave (phase offset) and the frequency difference (frequency offset) are zero. It is. Otherwise, non-zero phase and frequency offsets can appear in the received baseband signal, greatly affecting the performance of the coherent receiver. In an electrocoherent receiver, the phase offset and frequency offset are removed by precisely controlling the phase and frequency of the local oscillator with a phase locked loop. However, for optical coherent receivers, it is very difficult to precisely control the phase and frequency of the local oscillator by the phase-locked loop due to limitations of factors such as technology and standards related to optical devices. Therefore, in the optical coherent receiver, a new technique for eliminating the influence on the performance of the receiver due to the frequency offset and the phase offset of the baseband signal is required. At present, technologies capable of realizing the compensation of the phase offset in the baseband have already been developed, but the precondition for normal operation of these technologies is that the frequency offset is extremely small (MHz level).

  FIG. 1 shows an optical coherent receiver using a phase-locked loop. The phase offset detector 106, the loop filter 105, and the controller 104 in the figure constitute a phase locked loop. The output 111 of the phase locked loop controls the local oscillation laser 103 so that the frequency and phase of the output 102 of the local oscillation laser 103 coincide with the frequency and phase of the carrier wave. Further, the circuit including the optical frequency mixer 107 and the photoelectric converters (PD) 108 and 109 in FIG. 1 is defined as a front end circuit, and this front end circuit converts the input optical signal 101 into a baseband electric signal. Used to do. The data restorer 110 is used to restore the data signal. Due to limitations of factors such as technology and standards for optical devices, it is very difficult to implement the receiver shown in FIG. Thus, there is currently a technique that proposes processing the baseband digital signal to remove the phase offset.

  FIG. 2 shows an optical coherent receiver using a digital phase recovery technique. Compared to FIG. 1, FIG. 2 does not control the local oscillation laser 103, but instead uses a digital phase restorer 204 to remove the phase offset between the local oscillator and the carrier. The digital phase restorer 204 removes noise by averaging the phase offsets of multiple consecutive symbols to determine the actual phase offset, and uses this phase offset to correct the phase of those symbols to correct the phase. Remove offset. In FIG. 2, a front-end circuit including an optical frequency mixer 107, photoelectric converters 108 and 109, and analog / digital converters 201 and 202 is used to convert an optical signal into a baseband digital electrical signal 203. According to the above description, the operation principle of the digital phase restorer is to regard the phase offset of a plurality of consecutive symbols as a constant. However, when the frequency offset is not zero, the symbol phase offset changes with time. Therefore, the digital phase restorer cannot operate normally when the frequency offset is greater than several tens of MHz. On the other hand, in an actual optical communication system, it is affected by factors such as temperature, so that the frequency offset may reach several GHz. Therefore, frequency offset compensation is essential in optical coherent receivers that use digital phase recovery techniques.

FIG. 2 further shows an embodiment of a conventional digital phase restorer. However, the digital phase restorer can also be realized by other devices, that is, it only needs to restore the digital phase. As shown in the lower part of FIG. 2, the input of this digital phase restorer is a baseband electrical signal 203 output from the front end processing unit of the coherent receiver, that is, I + jQ = exp (jθ _d + jθ). . In the general case, the baseband electrical signal 203 includes not only the data information θ _d but also the phase offset θ between the carrier and the local oscillator. First, the baseband electrical signal 203 is input to the deflection angle calculator 207 to obtain the deflection angle 208, and the deflection angle 208 is input to the subtracters 223 and 210, respectively. The subtractor 210 subtracts the phase offset before N symbols fed back from the declination angle 208 to roughly obtain the data phase, and subsequently inputs the data phase to the 2π modulo calculator 211 and the quotient acquisition unit 212. The 2π modulo calculator 211 performs 2π modulo operation on the signal and limits it to 0 to 2π. The quotient acquisition unit 212 acquires the integer part of the quotient by dividing the signal limited to 0 to 2π by the 2π modulo arithmetic unit 211 by a predetermined value (for example, π / 2). Based on the result of the quotient acquisition unit 212, the signal 203 is input to the phase offset complex value acquisition unit 214, 215, 216, or 217. The phase offset complex value acquisition unit acquires the real part w2 and the imaginary part w1 of the phase offset 2 ^1/2 exp (jθ) displayed as a complex number, for example, using the equation shown in FIG.

Next, the noise removal device 219 removes noise in the estimated imaginary part and real part. The noise removal device may be realized by an averager, for example. An averager is a simple device that mathematically averages N symbols and may be used to remove noise. Subsequently, the declination calculator 221 calculates the declination θ of the restored prime number w ₂ + jw ₁ = 2 ^1/2 (cos (θ) + jsin (θ)) = 2 ^1/2 exp (jθ) after averaging. Is obtained and output, that is, the phase offset 222. The subtractor 223 subtracts the phase offset 222 from the signal 208 and outputs the result as a restored digital phase. At the same time, the phase offset 222 is further passed through the delay unit 213 to generate a delay of N symbols, and then supplied to the subtractor 209 for next use.

  For convenience of explanation, the subtractor 223 may be regarded as a phase restorer, and other components (that is, a declination calculator 207, a subtractor 210, a 2π modulo calculator 211, a quotient acquisition unit 212, a phase offset complex value acquisition unit) 214 to 217, noise removal device 219, declination calculator 221 and the like) may be regarded as a phase offset calculation unit.

  In view of the above prior art, an object of the present invention is to provide a frequency offset compensating apparatus and method and an optical coherent receiver for solving one or more problems due to the limitations and disadvantages of the above prior art.

  According to one aspect of the present invention, there is provided a frequency offset compensator built in a phase offset compensator of an optical coherent receiver. The optical coherent receiver includes a frequency offset estimation device, and the frequency offset estimation device estimates a change in phase offset due to a frequency offset of a baseband digital electric signal of the optical coherent receiver. The frequency offset compensator integrates a change in the phase offset to obtain a reciprocal of the phase offset in a single symbol time, and the change in the phase offset is an average of the phase offset compensator. A multiplier that multiplies the length, and a second integration unit that integrates an output of the multiplier and obtains a reciprocal of the output of the multiplier.

  The present invention provides a frequency offset compensation apparatus and method, and an optical coherent receiver.

1 is a diagram illustrating a conventional optical coherent receiver using a phase-locked loop. FIG. It is a figure which shows the conventional optical coherent receiver using a digital phase restoration technique. 1 is a diagram illustrating a frequency offset compensator according to an embodiment of the present invention. 1 is a diagram illustrating a frequency offset compensator according to an embodiment of the present invention. FIG. 3C is a diagram showing two implementation methods of the M output integrator shown in FIG. 3B. FIG. 3C is a diagram showing two implementation methods of the M output integrator shown in FIG. 3B. It is a figure which shows the frequency offset compensation apparatus by another Example of this invention, and the digital phase restoration apparatus which integrated this apparatus. It is a figure which shows the optical coherent receiver using the frequency offset compensation apparatus of this invention. It is a figure which shows the optical coherent receiver using the digital phase restoration apparatus which integrated the frequency offset compensation apparatus of this invention. It is a figure which shows the other optical coherent receiver using the digital phase restoration apparatus which integrated the frequency offset compensation apparatus of this invention. It is a figure which shows the other optical coherent receiver using the frequency offset compensation apparatus of this invention. It is a figure which shows the optical communication system using the optical coherent receiver of this invention.

  Next, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In addition, these Examples are only illustrations and do not limit this invention.

FIG. 3 shows a frequency offset compensator according to the first embodiment of the present invention. FIG. 3A shows the serial configuration of this device, and FIG. 3B shows the parallel configuration of this device. In FIG. 3A, input 203 is a received baseband digital signal. When there is a frequency offset, the input 203 may be expressed as I + jQ = exp (jφ _d + jφ + jkΔωT), where φ _d is data information, φ is the laser line width, etc. Is the phase offset, Δω is the frequency offset, T is the symbol period, k is the time sequence of symbol arrival, kΔωT is the phase offset due to the frequency offset, and j is the imaginary unit. The input 303 is a phase offset change (that is, ΔωT) due to the frequency offset within one symbol period estimated by the frequency offset estimation apparatus. The multiplier 304 multiplies the input 303 by −1 to obtain the inverse 305 of the input 303. The adder 306, 2π modulo arithmetic unit 307, and register 308 of FIG. 3A constitute an integrator, and the output 310 of this integrator is the reciprocal of the phase offset due to the frequency offset (ie, θ = −kΔωT). The output 310 is supplied to the complex multiplier 311. The complex multiplier 311 generates a complex number exp (jφ) based on the output 310 and multiplies the input 203 to obtain an output 304. This complex multiplication can be expressed as:

I ′ + jQ ′ = (I + jQ) exp (jQ) = exp (jφ _d + jφ + jkΔωT) × exp (-kΔωT) = exp (jφ _d + jφ)
As shown in the above equation, the phase offset due to the frequency offset is no longer included in the deflection angle of the output 304 (ie, I ′ + jQ ′).

FIG. 3B shows a parallel implementation of the frequency offset compensator of the first embodiment of the present invention. The parallel frequency offset compensator 312 includes a serial-to-parallel converter 326, a parallel-to-serial converter 330, a multi-output integrator 335, and M complex multipliers. M is the number of circuits processed in parallel. The serial-to-parallel converter 326 converts the input continuous baseband digital signal into an M-branch parallel output. For example, the first branch 336 is the first symbol and the second branch 337 is the second branch. Symbol. The outputs of the M branches are respectively supplied to M complex multipliers such as the complex multipliers 327, 328, and 329 in the figure. Input 303 is the change in phase offset with frequency within one symbol period. Multi-output integrator 335 performs calculation based on input 303 and outputs the reciprocal of the phase offset due to the frequency offset of the phase of each symbol. For example, the illustrated outputs 332, 321, and 324 are the reciprocals of the phase offset due to the frequency offset of the first symbol, the second symbol, and the Mth symbol, respectively. The M outputs of the multi-output integrator 335 are also supplied to M complex multipliers, respectively. Each complex multiplier generates a complex number exp (jQ _i ) (where i is 1 to M) based on the input phase offset, and also multiplies the input symbol, thereby changing the phase of the frequency offset symbol. To compensate for the effects of The output of each complex multiplier is combined into a serial continuous output of one branch after passing through the parallel-serial converter 330.

FIG. 4 shows two implementations of the multi-output integrator shown in FIG. 3B. As shown in FIG. 4A, the multi-output integrator according to the first method employs a cascade connection of adders. Specifically, the input 303 is multiplied by −1 to obtain a signal 314, and the signal 314 is supplied to N adders, for example, adders 315, 319, and 322 shown in the figure. The first adder 315 adds the signal 314 and the output 318 of the register 316 to obtain a signal 331. The 2π modulo calculator 317 limits the value of the signal 331 to the interval [0, 2π]. The output 332 (ie, θ ₁ ) of the 2π modulo operator 317 is supplied to the complex multiplier 327 to compensate for the frequency offset of the first symbol. At the same time, the output 332 is supplied to the second adder 319. The adder 319 adds the output 332 to the signal 314, and the modulo calculator 320 performs 2π modulo operation to obtain the signal 321 (θ ₂ ). Then, the signal 321 is supplied to a complex multiplier 328 to compensate for the frequency offset of the second symbol. By analogy with this, the reciprocal of the phase offset due to the frequency offset of each branch is obtained by the combination of each adder and modulo calculator, and is input to the corresponding complex multiplier. The output 324 of the last modulo calculator 323 is used to compensate for the frequency offset of the Mth symbol, while the output 324 is provided to a register 316 to compensate for the M + 1th symbol. Prepared for. The combination of each adder and modulo calculator may be referred to as an integration unit.

  As shown in FIG. 4B, another implementation of multi-output integrator 335 multiplies input 303 by -1 to -M, respectively (or multiplies the result of multiplying input 303 by -1 by 1 to M, respectively. To determine the phase offset change relative to the previous M-th branch symbol due to the frequency offset of the phase from the first branch to the M-th branch symbol, and the multiplier outputs 345, 346, 347 and the like are added to the phase offset of the previous M-th symbol (which is stored in the register 351) by the adders 338, 339, and 343, respectively, so that the symbols of the first branch to the M-th branch are added. It is to obtain phase offsets 348, 349, 350, etc. due to frequency offset in the phase. These phase offsets are input to 2π modulo calculators 340, 341, 344, etc., respectively, and the range of their values is limited to the interval [0, 2π]. As a result, signals 332, 321 and 324 for compensating the frequency offset are obtained.

  The frequency offset compensator shown in FIG. 3A needs to complete all the operations in FIG. 3A within one symbol period, which greatly increases the demand for hardware processing speed. On the other hand, the parallel realization of the frequency offset compensator shown in FIG. 3B is completed by assigning the processing to N symbols to N hardware, and each hardware is within its N symbol periods. It only needs to complete the process. This makes it possible to reduce the hardware processing speed requirement N times on the premise that the performance of the compensation is not affected. This makes it very convenient to implement the hardware in the frequency offset compensator.

ここで、Ｎは、デジタル位相復元装置の平均長さであり、n_kはノイズである。その直後にあるＮ個のシンボルに対して、位相オフセット２２２の値は、下式で示すようになる。

上記分析によれば、周波数オフセットの存在は、一方では、時間の変化に伴って変わる位相オフセットがシンボルの偏角に取り込まれるからであり、即ち、各々のシンボルの位相オフセットがそれぞれその前のシンボルよりkΔωTを増加することであり、他方では、時間の変化に伴って変わるオフセットがＮ個のシンボルの各々への推定による位相オフセットに取り込まれるからであり、即ち、Ｎ個のシンボルの各々の位相オフセット推定値のオフセットがその前のＮ個のシンボルよりNΔωTを増加することである。本発明の集積周波数オフセット補償装置の原理は、位相を復元すると同時に、周波数オフセットによる、前記二つの方面におけるオフセットを除去することにより、周波数オフセットによる受信機の性能への影響を補償することにある。 FIG. 5 shows a frequency offset compensator according to another embodiment of the present invention. The frequency offset compensator shown in FIG. 3 should be positioned in front of the phase restoration device, that is, to restore the phase after compensating for the frequency offset. On the other hand, the frequency offset compensator of FIG. 5 is integrated in the digital phase restoration device, and realizes frequency offset compensation while restoring the phase. The digital phase restoration device in FIG. 5 is a digital phase restoration device 401 in which the frequency offset compensation device 407 of the present invention is integrated. In order to explain the operation principle of this apparatus, first, the influence of the frequency offset on the digital phase restoration apparatus shown in FIG. 2 will be analyzed. If the frequency is Δω, the input baseband digital signal can be displayed as (I + jQ) exp (jQ) = exp (jφ _d + jφ + jkΔωT). The output 208 of the deflection angle calculator 207 in FIG. 2 is φ _d + φ + kΔωT. Further, the estimated phase offset 222 can be expressed by the following equation.

Here, N is the average length of the digital phase restoration device, and n _k is noise. For the N symbols immediately after that, the value of the phase offset 222 is as shown in the following equation.

According to the above analysis, the presence of a frequency offset is because, on the one hand, a phase offset that changes with time is taken into the declination of the symbol, ie, the phase offset of each symbol is the symbol preceding it. KΔωT is increased, and on the other hand, the offset that changes with time is taken into the phase offset by the estimation to each of the N symbols, ie the phase of each of the N symbols. The offset of the offset estimation value is to increase NΔωT from the previous N symbols. The principle of the integrated frequency offset compensator of the present invention is to compensate the influence of the frequency offset on the performance of the receiver by restoring the phase and removing the offset in the two directions due to the frequency offset. .

  The input 303 of the frequency offset compensator 407 according to this embodiment of the present invention is the output of the frequency offset estimator, that is, the phase change (ΔωT) due to the frequency offset within the period of one symbol. This input 303 is supplied to two branches, and one branch is an integrator (first integrator) composed of an adder 411, 2π modulo calculator 408 and a register 410, and an output 403 (first integrator) of this branch. Integration output) is the phase offset of the symbol with frequency (ie, kΔωT). The other branch is multiplied by N in multiplier 412 (N is the average length of the digital phase recovery device, which is one control parameter of the digital phase recovery device), and The other integrator (second integrator) including the adder 414, the register 417, and the 2π modulo calculator 418 is input. The output 404 (second integration output) of the second integrator is an offset (that is, NΔωT) of the phase offset estimated value by frequency.

A digital phase restoration device 401 in FIG. 5 is a digital phase restoration device in which a frequency offset compensation device 407 is integrated. By subtracting one output 403 of the frequency offset compensator 407 from the output of the deviation calculator 207, the symbol deviation 405 from which the phase offset due to the frequency offset has been removed is obtained, and the frequency offset compensation is obtained from the phase offset estimated value 222. By subtracting another output 404 of the device 407, the estimated offset due to the frequency offset is removed and the phase offset declination 406 is obtained. By performing subtraction on argument 405 and 406 by subtractor 223, to acquire data information phi _d.

  As can be seen by comparing FIG. 2 and FIG. 5, the present invention only requires the addition of the first subtractor 402 and the second subtractor 429 to the conventional digital phase restorer. The subtractor 429 subtracts the second integral output 404 of the frequency offset compensator 407 from the phase offset calculation result of the conventional digital phase offset restorer, and feeds back the phase offset obtained by subtracting the second integral output to the phase offset calculator. In addition, the output is subtracted from the output of the phase restorer 223. The subtractor 402 subtracts the first integrated output 404 of the frequency offset compensator 407 from the declination calculated by the declination calculator 207 and transmits it to the phase restorer 223. This can thus be realized easily. In this case, the subtractor 402 and the subtractor 409 can be used as the first interface and the second interface of the present invention, respectively.

  Further, as is well known to those skilled in the art, the embodiment shown in FIG. 5 is merely illustrative and does not limit the present invention. For example, the second integration output may be divided into two branches and input directly to the delay device 213 and the second subtracter 223 shown in the figure, respectively. Further, the first integrated output may be directly output to the subtracter 223 and the subtractor 210, respectively. In this way, there is no need to add other parts. In this case, the port of the subtractor 210 for receiving the first integral output of the frequency offset compensator 407 and the port of the phase restorer 223 for receiving the first integral output of the frequency offset compensator 407 are the first interface. Collectively, the port of the delay unit 213 for receiving the second integral output of the frequency offset compensator 407 and the port of the phase restorer 223 for receiving the second integral output of the frequency offset compensator 407 are the second interface. It may be generically named.

  2 and 5 are cited for convenience of explanation and are not intended to limit the present invention. In addition, any frequency offset calculation device known to those skilled in the art may be employed, that is, the first interface of the first integral output and the second integral output of the built-in frequency offset compensator 407 of the present invention, and the first interface. It only needs to have two interfaces.

  Compared with the frequency offset compensator of the first embodiment of the present invention shown in FIG. 3, the frequency offset compensator of FIG. 5 and the digital phase restoration device in which the device is integrated do not require complex multiplication. Can be greatly reduced. However, the drawback is that the compensation range is limited to the average length of the digital phase restoration device. If the rate of symbols to be transmitted is Br and the average length is N, the frequency offset range that can be compensated by this apparatus is [−Br / 2N, Br / 2N], while the frequency shown in FIG. The offset compensator can realize compensation for an arbitrary frequency offset.

  FIG. 6 shows an optical coherent receiver using the frequency offset compensator of the present invention. Compared to FIG. 2, the receiver includes a frequency offset estimator 501 and a frequency offset compensator 502 before the digital phase restorer 204. The frequency offset estimator 501 is a frequency offset estimation device proposed by A. Leven et al. In 2007 (“Frequency Estimation in Intradyne Reception”, IEEE Photonics Technology Letters, Vol. 19, No. 6, March, 2007, pp366-368 ) May be realized. Further, the frequency offset compensator 502 may be realized by the frequency offset compensator 302 in FIG. 3A or the frequency offset compensator 312 in FIG. 3B.

  FIG. 7 shows an optical coherent receiver incorporating the digital phase restoration device 401 of the frequency offset compensator 407 of the present invention. This receiver employs the frequency offset estimator 501 in FIG. 6, and the output of the frequency offset estimator 501 is transmitted to the digital phase restoration device 401 incorporating the frequency offset compensator 407. The digital phase restoring device 401 compensates the frequency offset at the same time as restoring the phase.

  FIG. 8 shows another optical coherent receiver using the frequency offset compensator 407 of the present invention and the digital phase restoration device 401 incorporating the device. The coarse frequency offset monitoring device 703 in the figure may be realized by a frequency offset monitor disclosed in a Chinese patent specification having an application number 200710139796.1 filed by Tohshinning et al. Since the polarity of the output of the frequency offset monitor changes with the positive / negative change of the frequency offset, the output of the frequency offset monitor may be regarded as a feedback signal for controlling the frequency of the local oscillation laser. . In addition, since this monitor has a strong waveform distortion tolerance of the received signal, it is possible to tolerate problems such as high chromatic dispersion and polarization mode dispersion and non-synchronized clocks in transmission / reception data. However, the problem with this monitor is that if the frequency offset is less than a certain range (eg, 1 GHz), the monitor will generate an erroneous monitor signal. As a result, there may be residual frequency offset in the baseband digital signal. Therefore, the frequency offset compensator 407 of the present invention and the digital phase restoration device 401 incorporating the device are applied to the optical coherent receiver, and the frequency offset remaining in the baseband digital signal due to the error of the coarse frequency offset monitor 703. To compensate. One of the two inputs of the digital phase restoration device 401 is a baseband digital signal. This signal may have already been processed by the digital equalizer 701, and the function of the digital equalizer 701 is to equalize signal distortion caused by chromatic dispersion and polarization mode dispersion in the channel. It is. The digital equalizer 701 is a conventional technique (eg, “Adaptive digital equalization in the presence of chromatic dispersion, PMD, and phase noise in coherent fiber optic system”, Crivelli, DE et al., Global Telecommunications Conference, 2004, Page ( s): 2545-2551 Vol.4) can be realized. The frequency offset estimator 501 calculates a residual frequency offset 303 in the baseband signal based on the baseband digital signal 203 output directly by the ADC or the baseband digital signal 506 after equalization output by the digital equalizer 701. presume. The digital phase restorer 401 uses the input 302 to restore the phase and at the same time compensate for the residual frequency offset. Compared to FIG. 7, FIG. 8 adds the initial control of the frequency offset by a feedback control loop composed of a coarse frequency monitor or the like, thereby reducing the compensation range of the digital phase restoring device 401. Can be supplemented on the mechanism.

  Although FIG. 8 shows the frequency offset compensation device 407 and the digital phase restoration device 401 incorporating the device, the phase offset and the restoration of the frequency offset are the same as those of the frequency offset estimation device shown in FIG. A combination with a phase restoration device may be employed.

  FIG. 9 shows another optical coherent receiver using the frequency offset compensator of the present invention. The frequency offset compensator 801 in the figure may be realized by the frequency offset compensator 302 or 312 in FIG. The input of the frequency offset estimator 501 in FIG. 9 is a signal after frequency offset compensation, that is, the output 806 of the frequency offset estimator 501 is a residual frequency offset after frequency offset compensation, and this output 806 is integrated. Is input to the integrator 805, and is added to the previously estimated frequency offset in the register 808 by the adder 807 in the integrator 805 to obtain a new frequency offset 803. This new frequency offset 803 is supplied to the frequency offset compensator 801 for compensation. In this way, the frequency offset compensator 810, the frequency offset estimator 501 and the integrator 805 in FIG. 8 constitute a frequency offset compensation feedback loop.

  FIG. 10 shows an optical communication system using the optical coherent receiver of the present invention. The system includes a transmitter 901, a fiber optic link 906 and a receiver 905. The optical fiber link may include one or more nodes 902, an optical fiber 903, and an amplifier 904. All of the other components other than the optical coherent receiver 905 of the present invention may be configured by conventional techniques. For example, the transmitter technology disclosed in “Optical Differential Quadrature Phase-Shift Key (oDQPSK) for High Capacity Optical Transmission”, R.A.Griffin et al., OFC 2002 may be used.

  The apparatus of the present invention may be realized by a computer program. The computer program causes a computer or a logic device (for example, a field-programmable logic device) to realize the functions of the device or components, or a computer. Cause the corresponding method steps to be executed. The computer may be a general-purpose computer including a CPU, a ROM, an IO device, a hard disk, a RAM, or the like, or may be a dedicated computer. The computer program may be a single computer program or a program set composed of a plurality of computer programs.

  The computer program or computer program set can be stored in a computer-readable storage medium. The computer-readable storage medium may be any medium known to those skilled in the art, such as a CD, a DVD, a soft disk, a flash memory, and a magnetic tape.

  Moreover, the following additional remarks are disclosed regarding the embodiment including the above-described examples.

(Appendix 1)
A frequency offset compensator used in an optical coherent receiver,
The optical coherent receiver includes a front-end circuit and a frequency offset estimation device, and the front-end circuit is used to convert an input optical signal into a baseband digital electrical signal, and the frequency offset estimation device includes: Estimating a change in phase offset of the baseband digital electrical signal due to a frequency offset;
The frequency offset compensator is
The change of the phase offset due to the frequency offset estimated by the frequency offset estimation device is integrated, and M reciprocals of the phase offset due to the frequency offset (the reciprocal is a number obtained by reversing the positive and negative signs). M is an integer greater than 1, for example, and -a is called a reciprocal of a with respect to a),
A serial-parallel converter for converting the baseband digital electric signal into an M-parallel baseband digital electric signal;
The corresponding reciprocal number among the M reciprocal numbers output by the M output integrator is set to a complex number, and the corresponding parallel number among the M parallel baseband digital electric signals converted by the serial-parallel conversion device. M complex multipliers for multiplying the digitized baseband digital electrical signal;
A parallel-serial converter for converting the M parallel baseband digital electric signals multiplied by the complex multiplier into one baseband electric signal;
A frequency offset compensator.

(Appendix 2)
The frequency offset compensator according to appendix 1,
The M output integrator includes a multiplier, a plurality of integration units connected in series, and a delay unit;
The multiplier multiplies the phase offset change due to the frequency offset estimated by the frequency offset estimation device by −1 to obtain a reciprocal of the phase offset change due to the frequency offset, and the reciprocals are respectively Are input to each integration unit,
Each of the M integration units connected in series integrates an input phase offset change to obtain a phase offset due to the frequency offset,
The integration output of the last integration unit among the M integration units connected in series is input to the corresponding complex multiplier and simultaneously output to the delay unit, and the M integration units connected in series. The integration outputs of the other integration units are input to the corresponding complex multiplier and simultaneously to the next integration unit,
A frequency offset compensator that integrates the reciprocal number and the integral output of the last integral unit delayed by one symbol time by the delay unit among the M integral units connected in series. .

(Appendix 3)
The frequency offset compensator according to appendix 2,
Each integration unit includes an adder and a 2π modulo arithmetic unit connected in series, and the first adder among the M integration units connected in series has the reciprocal delayed by the delay unit. Adding the integration output of the last integration unit, and the 2π modulo calculator performs 2π modulo calculation on the addition result of the adder,
The adder of the integration unit other than the first of the M integration units connected in series adds the reciprocal to the output of the previous integration unit, and the 2π modulo arithmetic unit adds the addition result of the adder. Is a frequency offset compensator that performs 2π modulo operation.

(Appendix 4)
The frequency offset compensator according to appendix 1,
The M output integrator includes M parallel connected integration and delay units;
The output of the delay unit is input to each of the M integration units connected in parallel,
Each of the M integration units connected in parallel integrates a change in the phase offset due to the input frequency offset, and outputs a phase offset of each symbol due to the frequency offset, that is, an integral output. Each of the integral outputs is input to a corresponding complex multiplier,
The last integration unit among the M integration units connected in parallel, that is, the Mth integration unit, inputs the integration output to the delay unit, and the delay unit outputs the integration output for one symbol time. A frequency offset compensator for delaying and inputting to each integrating unit as an output of the delay unit.

(Appendix 5)
The frequency offset compensator according to appendix 4,
Each of the M integration units connected in parallel includes a multiplier, an adder, and a 2π modulo arithmetic unit. The multiplier of each integration unit multiplies by −1 to −M,
The adder of each integration unit adds the output of the multiplier to the output of the last integration unit among the plurality of integration units connected in parallel, and the 2π modulo arithmetic unit of each integration unit includes the addition A frequency offset compensator that performs 2π modulo operation on the addition result of the filter.

(Appendix 6)
A frequency offset compensator incorporated in a phase offset compensator of an optical coherent receiver,
The optical coherent receiver includes a frequency offset estimation device, and the frequency offset estimation device estimates a change in a phase offset due to a frequency offset of a baseband digital electric signal of the optical coherent receiver,
The frequency offset compensator is
A first integration unit that integrates a change in the phase offset to obtain a reciprocal of the phase offset of a single symbol;
A multiplier that multiplies the change in the phase offset by an average length of the phase offset compensator;
And a second integration unit that integrates an output of the multiplier and obtains a reciprocal of the output of the multiplier.

(Appendix 7)
The frequency offset compensator according to appendix 6,
The first integration unit and the second integration unit each include a loop adder, a 2π modulo arithmetic unit, and a delay unit connected in series,
The change in the phase offset is input to the adder of the first integration unit, the output of the multiplier is input to the adder of the second integration unit,
The output of the first integration unit and the second integration unit is an output of the 2π modulo calculator, and the delay unit delays the output of the 2π modulo calculator by one symbol.

(Appendix 8)
A phase restorer,
The frequency offset compensator according to appendix 6 or 7,
A first interface for receiving an output of a first integration unit of the frequency offset compensator;
A second interface for receiving an output of a second integration unit of the frequency offset compensator;
A phase offset calculation unit for calculating a phase offset from the input baseband electrical signal and the output of the first integration unit;
A phase restoration unit comprising: a phase restoration unit for restoring the data phase in the baseband electrical signal by subtracting the output of the first integration unit and the output of the second integration unit from the phase offset.

(Appendix 9)
An optical coherent receiver,
A front-end circuit that converts an input optical signal into a baseband electrical signal; and a frequency offset estimator that estimates a frequency offset in the baseband electrical signal;
The optical coherent receiver further compensates a phase offset due to the frequency offset of the baseband electrical signal based on the frequency offset estimated by the frequency offset estimator. An optical coherent receiver including a frequency offset compensator.

(Appendix 10)
The coherent receiver according to appendix 9, wherein
A coarse adjustment frequency offset monitoring unit for coarsely adjusting the frequency offset of the baseband electrical signal output by the front end circuit; and a digital equalizer for equalizing the baseband electrical signal output by the front end circuit; Further including
An optical coherent receiver is provided that estimates the frequency offset in the baseband electrical signal or the frequency offset in the baseband electrical signal equalized by the digital equalizer.

(Appendix 11)
The optical coherent receiver according to appendix 9, wherein
The optical coherent receiver further includes a digital equalizer and an integrator,
The front end circuit is connected to the frequency offset compensator; the frequency offset compensator is connected to the digital equalizer;
The digital equalizer equalizes the baseband electrical signal compensated by the frequency offset compensator, and inputs the equalized signal to the frequency offset estimator;
The frequency offset estimator estimates a frequency offset in a signal output by the digital equalizer, and outputs the frequency offset to the integrator. The integrator integrates the frequency offset, and integrates the frequency offset. An optical coherent receiver that outputs a later signal to the frequency offset compensator.

  The preferred embodiment of the present invention has been described above, but the present invention is not limited to this embodiment, and all modifications to the present invention are within the scope of the present invention unless departing from the spirit of the present invention.

101 Optical input 103 Local oscillation laser 104 Controller 105 Loop filter 106 Phase offset detector 107 Optical frequency mixer 108, 109 PD
110 Data Restorer 201, 202 ADC
204 Digital Phase Restorer 206 Data Restorer 335 Multi-Output Integrator 326 Series-Parallel Conversion 330 Parallel-Series Conversion 303 Frequency Offset Compensator 401 Digital Phase Restorer Having Frequency Offset Compensator 501 Frequency Offset Estimator 502 Frequency Offset Compensator 701 Equalizer 703 Coarse frequency offset monitoring device 706 Controller 801 Frequency offset compensator 805 Integrator 901 Transmitter 902 Node 903 Optical fiber 904 Amplifier 905 Coherent receiver

Claims (6)

A frequency offset compensator incorporated in a phase offset compensator of an optical coherent receiver,
The optical coherent receiver includes a frequency offset estimation device, and the frequency offset estimation device estimates a change in a phase offset due to a frequency offset of a baseband digital electric signal of the optical coherent receiver,
The frequency offset compensator is
A first integration unit that integrates the change in the phase offset to obtain a reciprocal of the phase offset in a single symbol time;
A multiplier that multiplies the change in the phase offset by an average length of the phase offset compensator;
A second integration unit that integrates the output of the multiplier and obtains the inverse of the output of the multiplier;
including,
Frequency offset compensator. The first integration unit and the second integration unit each include a loop adder, a 2π modulo arithmetic unit, and a delay unit connected in series,
The change in the phase offset is input to the adder of the first integration unit, the output of the multiplier is input to the adder of the second integration unit,
The outputs of the first integration unit and the second integration unit are outputs of the 2π modulo calculator, and the delay unit delays the output of the 2π modulo calculator by one symbol.
The frequency offset compensation apparatus according to claim 1. A phase restorer,
The frequency offset compensation device according to claim 1 or 2,
A first interface for receiving an output of the first integration unit of the frequency offset compensator;
A second interface for receiving an output of the second integration unit of the frequency offset compensator;
A phase offset calculation unit for calculating a phase offset from the input baseband electrical signal and the output of the first integration unit;
A phase restoration unit that subtracts the output of the first integration unit and the output of the second integration unit from the phase offset to restore the data phase in the baseband electrical signal;
Including a phase restorer. An optical coherent receiver comprising: a front-end circuit that converts an input optical signal into a baseband electrical signal; and a frequency offset estimator that estimates a frequency offset in the baseband electrical signal,
The frequency offset compensator according to claim 1 or 2, further comprising: a phase offset due to the frequency offset of the baseband electrical signal is compensated based on the frequency offset estimated by the frequency offset estimator.
Optical coherent receiver. A second frequency offset estimator that estimates a frequency offset of a baseband electrical signal output by the front end circuit, and adjusts an oscillator frequency using an output value of the second frequency offset estimator;
The optical coherent receiver according to claim 4. The optical coherent receiver further includes a digital equalizer and an integrator,
The front end circuit is connected to the frequency offset compensator; the frequency offset compensator is connected to the digital equalizer;
The digital equalizer equalizes the baseband electrical signal compensated by the frequency offset compensator, and inputs the equalized signal to the frequency offset estimator;
The frequency offset estimator estimates a frequency offset in a signal output by the digital equalizer, and outputs the frequency offset to the integrator. The integrator integrates the frequency offset, and integrates the frequency offset. The optical coherent receiver according to claim 4 or 5, wherein a subsequent signal is output to the frequency offset compensator.

Images